This invention relates to an induction motor and particularly to a stator winding for a polyphase induction motor.
Alternating current induction motors are widely used as driving sources. Three phase motors are widely used for the industrial applications. The motor consists of a stator having a three phase winding specially wound to develop a rotating magnetic field. A rotor is rotatably mounted within the stator magnetically coupled to the stator. The rotor may have a short circuited winding within which an induced field is created by energizing of the stator winding. The induced field tends to align with and follow the rotating field to create a rotating force and motion on the rotor as a result of the electromagnetic coupling between the stator rotating field and the rotor's induced field.
A polyphase motor includes a plurality of circumferentially distributed phase coils connected to different phases of an incoming power supply. The most conventional polyphase system in electrical power distribution is the three phase supply, wherein three alternating currents are generated spaced from each in time by 120 degrees. In the three phase motor, three different phase windings are provided one for each phase, with a plurality of coils, generally equally distributed, for each phase of the stator winding. Each coil generates a corresponding magnetic pole and includes a complementing coil unit for generating a complementing pole spaced approximately 180 degrees from the first.
The stator winding is wound of a low resistance wire. With a power supply connected directly to such a winding, a relatively high level current flows through the winding upon starting. During motor operation, the electromagnetic coupling and interaction limits the current flow from the power supply through the stator winding.
During the starting of the motor, however, the rotor is at an essential standstill and relatively slowly accelerates to operating speed. During this short period of time, the stator is essentially operating as a simple coil. As a result, relatively large amplitudes of current will be encountered, without special compensation introduced into the circuit.
Various systems and circuits have been developed to provide special circuit connections during the starting of the motor to minimize the current flow. Thus special winding connections are provided within the several coils of the motor which minimize the initial current surge. After the motor approaches or reaches operating speed, the circuit connection is automatically changed to provide system operation with maximum efficiency of windings connections.
A particulary satisfactory starting circuit which has been used consists of a double delta connected winding. In a delta winding, each phase winding constitutes one leg of an equi-triangle with the power supply connected respectively to the three apexes of the delta winding. When the motor operates in a double delta circuit connection, two coil pairs of each leg are connected in parallel for generating the corresponding north and south poles. During the starting of the motor, the pairs of windings of each phase are connected in a series circuit with the winding of another phase. The normal phase relationship is disrupted and the voltage is reduced on all coils. The impedance of the stator winding with this starting circuit connection is much higher and significantly minimizes the starting current surge.
Although the double delta circuit provides a highly satisfactory and effective current limiting circuit, the motor winding is relatively complex and assembly of the windings with the stator is relatively costly.
The stator for a polyphase motor is formed with a substantial plurality of winding slots. For example, a typical three phase motor of a four pole variety has each phase wound with coil pairs for generating the four poles consisting of two north and two south poles equicircumferentially distributed about the stator. The poles of the corresponding phases are shifted by 120 degrees. Generally, a plurality of coils such as three coils may be provided for establishing each pole. The coils are preferably prewound, introduced into the center of the stator and then moved outwardly into the appropriate slots. In conventional winding, various winding patterns have been developed, with different phase coils located in superposed relation within given slots. Thus any one slot will include the coil side of one winding in combination with the coil side of another winding.
A simpler winding system involves a special winding method identified as chain winding. In a chain winding, the coils are located in the stator slots with a single coil side located in each single slot. The winding is then connected in a special chain winding connection to develop the polyphase pole distribution. However, special current limit means must be provided during the starting and the chain winding is not particularly adapted to interconnection, for example, in the double delta start winding mode. For this reason, the chain type winding has not been used to the same degree as other types of winding where the motor size or power utility regulations require a reduction in starting current. Generally, the manufacturer elects a more complicated winding in order to obtain the improved starting characteristic within maximum overall motor efficiency at minimum overall cost.
There remains a distinct need in the art for a simplified winding system such as the chain-type winding with means for permitting the simplified winding method and apparatus permitted by the chain winding system.